Basics: The USA must economically conquest as much of South America as possible.  Since the US is a liberator, it will probably be a long time before it can conquer any enemy territories, therefore making other resources neccessary.  Added to this problem, the US has to buy lots of expensive ships and transports in order to move its expensive equipment.  Try and get an enemy territory in an area that you can bring the war to them from.  In this way, you will be able to devote your immense resources to defeating your enemy on the ground, rather than on equipment to move across the seas.

The Arsenal of Democracy
By General Beutin

    As America, you are the last one to be seriously affected by losses in territory and therefore have a large amount of money to spend on a steady basis.  First turn, economically conquest with your 7 IPC's and Lend-Lease the other 10 IPC's to Britain.  As you begin making your real money over the next few turns, Lend Lease as much as possible to Britain and Economically Conquest a lot of South America, espescially a straight line from Mexico to Brazil.  When you enter the war, always produce a transport and a plane every turn.  Work on technology and continue funding the other allies.  Be the arsenal of democracy.